Aim 1 Fluorescence resonance energy transfer (FRET), in which light energy absorbed by a donor is transferred to a nearby acceptor, is a powerful tool for measuring changes in molecular distances. The efficiency of FRET falls off with the sixth power of the distance between the two molecules, making FRET very sensitive to changes in distance. However, FRET can measure distances effectively only in a narrow range of distances that are not always well suited to study intramolecular movements in proteins. We are developing rapid high throughput methods that use transition metal ions (nickel and copper) as energy acceptors for small fluorescent donor dyes to map the conformational rearrangements of engineered proteins. These fluorescent methods work over shorter distances than classical FRET, use smaller dyes with shorter linkers, and are not as sensitive to the orientation problems usually associated with other methods. Aim 2 Membrane proteins in cells exist in a complex molecular environment. For example, many membrane proteins assemble as homomeric or heteromeric complexes. Furthermore, dozens of binding partners may transiently interact with membrane proteins to modulate their behavior or trafficking. Finally, the architecture of a protein is influenced by post-translational modifications and the native membrane environment. Understanding these complex structural parameters is necessary for understanding the function and regulation of membrane proteins. We are using FRET to map the structures of membrane proteins within native biological membranes. These studies will help us understand how these proteins are structured, how their complexes assemble, and how the trafficking of these complexes is regulated within living cells.